1. Field of the Invention
The present invention relates to a drowsiness determination apparatus, program, and method for determining a subject's drowsiness level.
2. Description of Related Art
In order to prevent a vehicle driver from dozing at the wheel, an apparatus has been proposed that captures a facial image of a subject, such as the driver, using a camera mounted in the vehicle. The captured image is processed to progressively determine a drowsiness level, for example, in a stepwise fashion.
Such an apparatus generally calculates a degree of eye opening of the subject from the captured image and determines a dozing state of the subject based on the calculation.
For example, a technology proposed in JP-1997-44685 A detects positions of an eyebrow and an eye and determines the degree of eye opening based on a relative distance between the eyebrow and the eye. The technology makes the determination based on an assumption that an increase in the relative distance between the eyebrow and the eye, such as an open portion of the eye or an upper eyelid indicates a decrease in the eye opening degree. The technology determines that the drowsiness level increases as the eye opening degree decreases.
When the above-mentioned technology gradually determines drowsiness levels based on eye opening degrees, it is necessary to settle multiple threshold values for the eye opening degrees and determine to which ranges separated by the threshold values the eye opening degrees correspond. However, there are differences among individuals in eye opening degrees and ways of closing eyes as the drowsiness increases. Settling a unique threshold value is therefore relatively difficult.
Further, individual actions of a subject may affect accuracy of such a method. For example, when drowsiness is realized, the subject may resist the drowsiness by deliberately raising the eyebrows so as to open the eyes wide. Further, differences in individual behavior and basic physical makeup result in variation in the relative distance between the eyebrow and the eye.
When the eyebrow is raised higher than, for example, a position associated with a normal state, the relative distance between the eyebrow and the eye becomes greater. As a result, the eyebrow-to-eye measurement becomes greater than the eyebrow-to-eye measurement associated with a decrease of the eye opening degree when the eyebrow is in the normal position. Accordingly, the drowsiness level is assumed to be higher than it is. Still further, when the drowsiness increases even further, the eyebrow lowers and thus, the relative distance between the eyebrow and the eye decreases, which would normally indicate a greater degree of eye opening even though the net eye opening degree may decrease. In such a case, the drowsiness level is assumed to be lower than it actually is.
For the above noted reason, it has previously been difficult to accurately determine the drowsiness level for the technology based only on the eye opening degree.